Germanium photodetectors may be employed in microphotonic devices for the high efficiency of photon absorption. Integration of a high quantum efficiency germanium photodetector into a silicon base semiconductor substrate faces a challenge because of the differences in material property in silicon and germanium.
For example, Yin at al., “40 Gb/s Ge-on-SOI waveguide photodetectors by selective Ge growth,” Optical Fiber Communication/National Fiber Optic Engineers Conference, pp. 1-3, February, 2008, Digital Object Identifier 10.1109/OFC.2008.4528025 discloses a silicon waveguide to which a Ge photodetector is attached. While the Ge photodetector in Yin provides an enhanced quantum efficiency over previous Ge photodetectors, the performance of the Ge photodetector is limited by alloying of the germanium material with the silicon material in the waveguide because germanium atoms have a high diffusivity in silicon. Since the photons in the silicon waveguide may be scattered or reflected even by small crystalline defects or impurities, such a direct contact between the silicon material in the waveguide and the germanium material in the photodetector has an adverse impact on the quantum efficiency. The wider the area of the contact between the silicon waveguide, the greater the amount of germanium atoms that diffuse into the silicon waveguide.
Further, silicon has a lattice constant of 0.543095 nm and germanium has a lattice constant of 0.564613 nm at 300 K. The resulting lattice mismatch of about 4% introduces severe strain on a germanium material when the germanium material is grown epitaxially on a silicon material. Such a strain in the epitaxially grown germanium generates crystalline defects, which generates a significant amount of dark current in the germanium photodetector. The dark current is the electrical current that a photodetector generates in the absence of any signal, i.e., in the absence of any light signal in the silicon waveguide. A high dark current makes distinction between presence and absence of light signal in the silicon waveguide difficult.
In view of the above, there exists a need for a germanium photodetector for detecting light in a silicon waveguide with high quantum efficiency and a minimal amount of dark current, and methods of manufacturing the same.
Specifically, there exists a need for a germanium photodetector that minimizes introduction of a germanium material into a silicon waveguide as well as minimizing crystalline defects in the germanium material of the photodetector, and methods of manufacturing the same.